Jack, extract from the OP
A lot has happened since then. Nikon did come out with a similar lens that is as good and 0.8 kg lighter. Sony came out with the 200-600mm, and Art Morris has gone over to Sony. The facts are that none of the C, N and S are produce perfect all round systems for us birders and all three have weaknesses as well as pluses. I need lighter gear and the 1DXIII and 400mm DO II are too heavy for me now. Sony makes the only mirrorless with AF good enough for serious BIF and now the best, but too heavy for me, zoom. Canon makes for me the best lighter zoom, the 100-400mm II, and pretty good prosumer DSLRs. Nikon makes the best prosumer FF and APS-C DSLRs for AF with excellent IQ and fps but only one Nikkor lens I want. I now shoot across makers because of this, and the jury has yet to see the evidence if the R5 and new lenses will fulfil our needs.
This is not a whinge, quite the reverse. I am really happy with the choice of gear and am able to take much better photos now than I could have dreamed about a few years ago.